This invention relates to oxygen-permeable polymeric membranes to be used in processes for producing oxygen- or nitrogen-enriched air for industrial, medical, and other applications. More particularly, the invention concerns polymeric membranes which contain, as dispersed therein, a metal complex capable of adsorbing and desorbing oxygen rapidly and reversibly.
Oxygen is one of the chemicals most widely used on industrial scales, specifically in the manufacture of iron, steel, and other metals and glass, in chemical oxidation and combustion, and in wastewater disposal. It has also very extensive usage in the field of medical care, including the therapy for lung disease patients by means of oxygen inhalation. Nitrogen, on the other hand, is a chemical conveniently and extensively used to maintain a nitrogen atmosphere, for example, for the preservation of foods, in fermentation processes, and in electronic circuit fabrication. For these reasons the development of processes for concentrating oxygen and nitrogen out of air is an important problem with far-reaching effects on various sectors of industry. While low-temperature separation and adsorption are in use as industrial processes for the concentration of oxygen and nitrogen from air, membrane separation is considered promising from the energy-saving viewpoint.
Success of membrane separation depends primarily on the discovery of a membrane material that would permit selective and efficient oxygen permeation relative to nitrogen from air. Currently available membranes capable of permeating and concentrating atmospheric oxygen (known as oxygen-permeable membranes) are those of silicone, silicone polycarbonate, and the like. Some of them are in practical service. They do not have high oxygen-permeation selectivity (O.sub.2 /N.sub.2) value (oxygen-permeability coefficient/nitrogen-permeability coefficient), the value being approximately 2, and yet exhibit high permeability coefficient (10.sup.-8 [cm.sup.3 .multidot.(STP).multidot.cm/cm.sup.2 /.multidot.sec.multidot.cmHg]). With this feature the membranes are incorporated in modules, multistage processes, and other systems to obtain oxygen-enriched air, with oxygen concentrations of approximately 30%. In order to obtain highly oxygen-rich air useful for industrial and medical applications by a single, continuous permeable-membrane pass, it is essential that the separating membrane have an (O.sub.2 /N.sub.2) value of at least 5.
The first requisite for an enhanced selectivity (O.sub.2 /N.sub.2) is to make oxygen more soluble than nitrogen with respect to the membrane.
We have hitherto continued the synthesis of metal complexes capable of rapid, reversible adsorption and desorption of oxygen molecules. As a result, we clarified essential requirements of the metal complexes that can adsorb and desorb oxygen molecules selectively, rapidly, and reversibly, even in a solid-phase membrane polymer. We successfully synthesized the novel complexes and taught their use for oxygen-enriching membranes (Japanese Patent Application Public Disclosure No. 171730/-1987).
Highly oxygen-rich air is useful for industrial and medical applications, and large quantities of highly nitrogen-rich air are used as inert gas in many sectors of industry. If they are to be obtained continuously by a single economical pass, it is essential that the separating membrane have a selectivity (O.sub.2 /N.sub.2) value of at least 5 and a membrane life long enough to maintain the performance stably for months.
We have hitherto carried on the synthesis of metal complexes capable of rapid, reversible adsorption and desorption of oxygen molecules. As a result, we successfully synthesized novel metal complexes that can adsorb and desorb oxygen molecules selectively, rapidly, and reversibly, even in a solid phase. We further found that the metal complexes carried in high-molecular-weight solid-phase membranes are kept from irreversible oxidation and permit stable, selective permeation of oxygen.
However, polymeric membranes incorporating such complexes, when used in air permeation, did not always achieve the object satisfactorily. Although the (O.sub.2 /N.sub.2) value exceeded the target value of 5, the permeability dropped to 50% in no more than 20 days of continuous permeation.